The present invention relates generally to infrared detectors, and more particularly to a method of compensating for non-uniformities among detector elements of an infrared detector array.
Infrared detectors provide thermal images for temperature measurement and heat detection. They are used for various applications, such as for military, industrial, and medical applications. In its simplest form, an infrared detector is a device, such as a photosensitive diode, that generates an electric current when exposed to infrared radiation. This current is dependent on the intensity and wavelength of the radiation and can be used in many different ways to produce an infrared picture.
Infrared detectors may be configured as a single element (detector), a small array of elements, a long linear array, or a full two-dimensional array. When the detector is a full two-dimensional array, the entire image is recorded at once, and the array is referred to as a xe2x80x9cstaringxe2x80x9d array. However, with smaller arrays, the image is scanned over the array. The small array requires a serial scan to sweep the image in two-dimensions, whereas the linear array requires a xe2x80x9cpushbroomxe2x80x9d scan to sweep the image across the array in one dimension.
The current produced by an infrared detector is amplified and processed to provide a more useful detector output. The processing reduces interference due to external and internal causes, such as electrical noise.
The ideal response of an infrared detector array is that each detector element exhibit the same linear voltage response for given temperature changes in the irradiation of the array. However, one type interference with a good detector signal is electrical noise due to detector non-uniformity among detector elements. The uniformity differences have both spatially and temporally dependent causes.
A number of methods have been tried for compensating non-uniformity of infrared detector arrays. Generally, all involve some sort of data processing. Some methods use a uniform calibration source, typically using a chopper and controlled temperature. Other methods are scene-based, which means that they use an image comprised of one or more objects or patterns. The scene-based methods may be further categorized into mechanical and non-mechanical methods.
Mechanical methods include methods that use choppers, dither mirrors, or other devices to blur the scene or otherwise induce motion. The xe2x80x9cdithered scanxe2x80x9d method of non-uniformity compensation is a scene-based mechanical method. The detector array views a scene through suitable optics. During a given time frame, the incident flux is sensed by each detector element. At the end of the time frame, the array data is delivered for processing and the array is displaced (xe2x80x9cditheredxe2x80x9d) a fixed distance, typically a distance equal to the width or height of one detector element, in either the horizontal or vertical direction. Conventional dither scan methods assume the scene flux to be stable throughout the dither cycle. Thus, during the next time frame, each detector element is exposed to the flux seen by one of its neighbors during the prior time frame. These detector pairs can be xe2x80x9clinkedxe2x80x9d analytically, such as by averaging their outputs. By a suitable choice of a dither pattern, each detector can be linked with one or more of its neighbors, to adjust gain and offset differences. Dithered scan methods are described in U.S. Pat. No. 5,925,880, to C. J. Young, et al, entitled xe2x80x9cNon-Uniformity Compensation for Infrared Detector Arraysxe2x80x9d.
Scene-based non mechanical methods are based on continuous scene or platform motion. These methods have included temporal high pass filtering, neural networks, and constant statistics.
One aspect of the invention is a ditherless method of compensating non-uniformities among detector elements of an infrared detector array. It is first determined whether there is relative motion of the scene and the detector array. If so, a spatial low pass filter type of non-uniformity compensation (NUC) algorithm is performed. This NUC algorithm is characterized by its use of neighborhood averaging to adjust offset differences. The neighborhood averaging uses the sum of xe2x80x9cshifted image differencesxe2x80x9d, where shifted images (matrices of detector output values) from a first field and a second field, respectively, are differenced. This sum is divided by four.
The NUC algorithm has various mathematical equivalents. However, these equivalents are common in the sense that a detector element and its neighbors are exposed to scenes that differ due to motion. The algorithms calculate a local response average, which is subtracted from the detector element""s output to determine the offset correction for that detector element.
The method is iterative, such that new offset correction updates approach zero, reducing offset errors to smaller and smaller errors. The image can be displayed during the iterations with successively smoothed offset errors, or the display can be delayed until the correction values become stable to some predetermined level.
An advantage of the invention is that it permits use of the same algorithms as are used for dithered scan compensation. With sufficient scene motion, the low pass filter effects of the algorithms remove the fixed pattern noise created by non-uniformities, without the need to dither the detector array. Experimentation with a detector system on board an aircraft has shown that the algorithms are sufficient, without dithering, to remove fixed pattern noise at ground scene, where there is large frame to frame scene motion, as well as at sky, where there is small frame to frame scene motion. At the same time, the image of a dim moving target at distance is preserved.